Electrochemical sensors traditionally comprise a gas diffusion working electrode, often based on a platinum or graphite/platinum catalyst dispersed on polytetrafluorethylene (PTFE) tape. The target gas is reacted at this electrode while a balancing reaction takes place at the counter electrode. The electrodes are contained within an outer housing which contains a liquid electrolyte, such as sulfuric acid. The gas typically enters the housing through a controlled diffusion access port, which regulates the ingress of target gas into the cell. The gas reacts at the electrode and affects the electrical output of the sensor.
Conventional electrochemical gas sensors mostly employ aqueous solutions of acids (typically sulfuric acid) as the electrolyte. Under benign environmental conditions and short excursions into extreme environments, the composition of the electrolyte remains fairly constant and the sensor performance exhibits minimal deviation from calibration. However, prolonged subjection to severely hydrating (high relative humidity, (RH %)) or dehydrating conditions (low RH %) leads to equilibration of the electrolyte with the environmental relative humidity, and consequently a change in the composition of the electrolyte. This manifests as a deviation in sensor performance from calibrated values or, in extreme instances, the failure of the sensor.
Due to the small quantities of acid incorporated in small sized sensors, it is clear that even small amounts of water exchange between the sensor and the environment has the potential to affect the performance to a degree that would be considered unsatisfactory. Water evaporation from the electrolyte is also problematic. It is desirable for the sensor's working lifetime to be as long as possible but moreover it is important that any particular sensor type will consistently continue to work for at least the indicated lifetime. Early failures lead to the need for more frequent sensor replacement, as well as increased monitoring of sensor performance and, ultimately, a loss in confidence in the sensor. Accordingly, there is a need to produce sensors that have a longer lifetime, which maintain an acceptable level of performance under many different operating environments.